1. Field of the Invention
The present invention relates to a positioning device for positioning a table. The present invention also relates to a lithographic apparatus using the positioning device, and a device manufacturing method.
2. Description of the Related Art
FIG. 7 is a schematic diagram showing the configuration of a positioning device employed in a conventional lithographic apparatus. The following positioning device is based on the assumption that a linear motor is used as the driving unit for driving a table. A table 101 places various originals such as a reticle or various substrates such as a wafer for performing linear driving. The table 101 is mounted on a base 102, and the base 102 includes a guide (not shown) for guiding the table 101 in a predetermined axial direction. The distance over which the table 101 is moved on the guide is output by a position sensor 103 as a pulse signal. The pulse signal output from the position sensor 103 is integrated by a counter 104 so as to output the current position signal of the table 101.
A phase switch controller 105 outputs a coil selection signal to coil selection switches 106a to 106d, based on the current position signal from the counter 104. The coils 112a to 112d are connected to electric current amplifiers 108a to 108d, respectively, via the coil selection switches 106a to 106d, respectively. The phase of the coils 112a to 112d is switched by switching a electric current command value from a servo controller 107 depending on the coil selection signal. The servo controller 107 calculates the difference between the target position from an upper-level controller (not shown) and the table 101's current position from the counter 104 so as to perform control calculation such as position servo control or speed servo control. Furthermore, the servo controller 107 outputs a electric current command value calculated by the control calculation to the electric current amplifiers 108a to 108d. The electric current amplifiers 108a to 108d supply a drive electric current depending on the electric current command value from the servo controller 107 to the coils 112a to 112d, respectively. The coils 112a to 112d are fixed to a coil support member (not shown), and a linear motor is constituted by the coils 112a to 112d together with magnets (not shown) securely fixed to the table 101 in a predetermined arrangement. The linear motor causes a predetermined electric current to flow to the coils 112a to 112d in a predetermined order so as to drive the table 101 in the axial direction.
Braking switches 110a to 110d are respectively disposed between the coils 112a to 112d of the linear motor and the electric current amplifiers 108a to 108d. Here, the braking switches 110a to 110d typically connect the coils 112a to 112d to the current amplifiers 108a to 108d, respectively. When a braking signal, i.e., an emergency stopping signal from the upper level controller is input to a brake controller 109, the brake controller 109 switches all of the braking switches 110a to 110d to the opposite contact points. Then, both ends of all of the coils 112a to 112d are short-circuited so as to activate a dynamic brake. These effects are disclosed in Japanese Patent Laid-Open No. 2001-85503. Japanese Patent Laid-Open No. 1-259405 discloses a mechanical brake as the other braking function of the positioning device. Furthermore, Japanese Patent Laid-Open No. 2007-114550 discloses a positioning device using a dynamic brake provided with a linear motor in order to impart a thrust force to a table.
In order to stop the table 101 by means of a strong braking force, control driving such as position servo control, speed servo control, and the like is generally employed, and, it is assumed that all functions of the positioning device shown in FIG. 7 need to be operated in a normal way. For example, when the position sensor 103 does not operate normally due to a malfunction caused by a failure or noise, the control driving such as position servo or velocity servo described above cannot be used for braking. Accordingly, a brake function, which works even when any of the above functions of the positioning device are abnormal, becomes necessary. However, the braking force of the dynamic brake of the conventional positioning device is less than a fraction of that, as compared with a case where the table 101 is stopped using the control driving such as position control or velocity control. Hence, in recent positioning devices for which the acceleration and maximum speed have been increased to attain an increase in productivity, a conventional dynamic brake provides an insufficient braking force.
In addition, the dynamic brake has the characteristics that the braking force is weakened as the table speed decreases. Hence, when an emergency stop is carried out by the dynamic brake near the edges of the driving area on the base 102, the table 101 cannot be stopped within the driving area, resulting in a collision with the wall surface 113 of the base 102. In order to prepare for the collision, a shock absorber 114 has conventionally been provided on the wall surface 113 of the base 2. However, as described above, the acceleration and maximum speed of a table have been increasing year by year. Consequently, an impact upon collision with the shock absorber 114 has also been increased more than heretofore. Also, in order to accommodate an increase in the acceleration and the maximum speed of the table 101, the table itself is made a hollow structure for the purpose of weight reduction, so that a strong structure capable of undergoing a collision with the shock absorber 114 cannot be maintained. Thus, problems arise in that the table 101 itself may be destroyed due to an impact caused when the table 101 collides with the shock absorber 114.
Furthermore, the shock absorber 114 requires a large stroke, i.e., a large external dimension in order to absorb a violent impact as described above, which restricts the layout of the units of the exposure apparatus. Consequently, the size reduction of the exposure apparatus is prevented.
Moreover, as with the device disclosed in Japanese Patent Laid-Open No. 1-259405, a table provided with a brake mechanism increases the weight and the size thereof, whereby it becomes difficult to increase the acceleration and the maximum speed of the table. Consequently, an increase in productivity of the exposure apparatus may be prevented.